This invention relates to the destruction of hazardous waste and, more particularly, to the destruction of hazardous waste using an electrodeless radio frequency (RF) inductively coupled plasma torch.
A major problem facing modern society is the disposal of toxic waste materials in a manner which minimizes harmful effects on the environment. An ideal waste disposal system is one which is capable of reducing hazardous waste to compounds suitable for environmental disposal. Such suitability is, of course, defined in terms of acceptable levels of pollution as determined by a variety of regulatory agencies.
Traditionally, hazardous waste disposal has taken the form of direct burial in land fills, or thermal processing of the waste, followed by burial of the solid residue, and release to the atmosphere of the volatile residue. None of these approaches have proven acceptable, due to the fact that the materials released to the environment remain as unacceptable sources of pollution.
A number of attempts have been made in the prior art to destroy waste material using direct current (DC) arc discharge type plasma torches. One such attempt is disclosed in Boday, et al. U.S. Pat. No. 4,438,706. This reference teaches the use of a DC arc discharge plasma torch in combination with an oxidizing agent for the thermochemical decomposition of certain types of waste material. The torch gas is air, and the waste material in vapor form is introduced along with oxygen downstream of the plasma arc generator, where it is heated by the torch gas.
In Faldt, et al. U.S. Pat. No. 4,479,443, there is disclosed the use of an arc discharge plasma torch to thermally decompose waste material. Waste material in the form of solid particles must be introduced downstream of the arc to avoid fouling of the torch as a result of particle adherence. Oxidizing agents such as oxygen and air are mixed with the waste either before, during or after the waste is heated by the torch gas. Sufficient oxidizing agents are required for the complete oxidation decomposition of the waste material.
In Barton, et al. U.S. Pat. No. 4,644,877, there is disclosed the use of a DC arc plasma burner for the pyrolytic decomposition of waste. An organic fluid is used to start and stabilize the plasma arc, and annular electromagnetic field coils are used to collimate the plasma, and a high pressure air supply is used to spin the arc. Provisions are made for feeding waste material downstream of the arc electrodes to prevent interference with the formation or generation of the plasma arc. The reference teaches away from the use of an inert gas to initiate or sustain the plasma, on the basis that such a burner is only suitable for low temperature applications. A reaction chamber following the burner is used to combine gas and particulate matter, which is quenched and neutralized with an alkaline spray. A mechanical scrubber is used to separate gases, which are withdrawn using an exhaust fan.
Chang, et al. U.S. Pat. No. 4,886,001, discloses what is described as an improvement over the above-discussed system of Barton, et al. The improvement is the use of water or methanol in place of a miscible mixture of a solvent of MEK and methanol for combining with waste materials comprising PCBs prior to introduction into the DC arc type plasma torch, and the use of pure oxygen instead of air as the torch gas. The object of these changes is to increase the waste processing rate. Also disclosed is the use of a solid separator which employs a partial vacuum to separate carryover gases.
The prior art plasma waste decomposition systems suffer from a variety of shortcomings which have prevented their widespread use in commercial applications. One shortcoming results from the fact that the waste material generally cannot be introduced directly into the plasma arc because such introduction causes contamination of the arc electrodes and subsequent erratic operation of the arc. Thus, the waste material is introduced downstream of the arc an is indirectly heated by the torch gas. This technique shortens the high temperature residence time of the waste material, resulting in incomplete decomposition.
Further, the performance of the arc is highly sensitive to the waste and carrier gas flow rate. Thus, the flow rates must be confined within narrow limits, leading to difficulties in controlling and maintaining system performance. Arc electrode erosion with use further complicates the maintenance, operation, stability and safety of the system. Small scale operation of DC arc plasmas is also very inefficient due in part to the minimum gas flow rate and electric power requirements needed to strike and sustain the arc. Scaling the prior art systems for operation at different waste throughput levels and with a variety of waste materials has proven to be difficult, requiring major system configuration changes which are expensive to accomplish.
Additionally, the need for organic, oxidizing, and/or reducing agents to be combined with the waste material in the prior art systems often results in highly undesirable compounds in the waste residue.
In summary, none of the prior art systems have provided a method of reducing hazardous waste to compounds suitable for environmental disposal.